Parabolic Antenna Discussion

Piggie

Super Moderator
#2
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ProjectSHO89,
There are a number of websites with information on how to convert satelite dishes to receive UHF OTA. Here's one.
Jim

Building a Parabolic Reflector UHF Antenna Like A CM4251? - Digital Forum
I have heard about people that used the old large 3m C-band dishes as UHF OTA. Probably use some of the smaller new ones as well.

Too bad it's so expensive to set up an antenna jig. It would be great to bring back the UHF parabola OTA antennas. Then again they also lost the plans how to build a Saturn V.
 

Fringe Reception

Super Moderator, Chief Content Editor
Staff member
#3
LOL! Another memory!

Parabolic antennas do not have to be solid salad bowls - but this one was ... it was 1940s technology ...

... I always wanted one of them when I was a (young) teenager living at home. In the late 1960s I dragged home a surplused 4+ foot diameter dish from "Navy Mars" and my parents FREEKED! No way was that going on my parents' roof! RATS!

Three months later, the dish and its opposite/converse fiberglass cover were still on the ground in our front yard and there happened to be a major windstorm: Dad and I loaded them inside our family station wagon and we dumped them under a local antenna farm, thinking it was an obvious place for them to land during a storm: we assumed there would be some confusion within the three locals ... Ha ha!
Jim

Please check out the penultimate antenna of the day below:

Channel Master 4251 Tribute Page
 
Last edited:
#4
Too bad it's so expensive to set up an antenna jig. It would be great to bring back the UHF parabola OTA antennas.
It's not as hard to build a plane parabolic. Here's a commercially available design: http://www.kathrein-scala.com/catalog/PR-480.pdf I have one with a UHF TV feed. It outperforms my old 4228.

In order to get the parabolic shape you can bend aluminum tubing like a bow for an arrow. The shape will be parabolic by nature. No jig is needed. To get more gain, you can make the reflector wider and use multiple feed points fed in phase.

If you used 12' long tubing for the parabola and an 8' width lined with chicken wire and with 2 or 4 dipoles for the feed, the gain would be in the mid 20's.
 

Piggie

Super Moderator
#6
It's not as hard to build a plane parabolic. Here's a commercially available design: http://www.kathrein-scala.com/catalog/PR-480.pdf I have one with a UHF TV feed. It outperforms my old 4228.

In order to get the parabolic shape you can bend aluminum tubing like a bow for an arrow. The shape will be parabolic by nature. No jig is needed. To get more gain, you can make the reflector wider and use multiple feed points fed in phase.

If you used 12' long tubing for the parabola and an 8' width lined with chicken wire and with 2 or 4 dipoles for the feed, the gain would be in the mid 20's.
well on expensive I was quoting the article that Jim posted. Where when asked why they were not made any more th jig as the reason.

Now, that said you have a great way of homebrewing one! Thanks! :mad:)
 

BCF68

DTVUSA Member
#8
I used that pdf to show the concept. The exact antenna is here:
http://www.kathrein-scala.com/catalog/PR-TV.pdf
Says impedence is 50 OHMs. So how exactly do you hook it up to a TV? How to you go from 50 OHMs to 75 OHMs? Is there a 1.5:1 balun I'm not aware of?

Anyways seems to have the best gain of any antenna I've seen. So how much is one of those things?

Also what would happen say you had an old satelite dish. You know HUGE kind people had before directv and put an antenna where the LBN goes? Assuming you could aim it at a station.
 

EscapeVelocity

Moderator, , Webmaster of EV's Antenna Blog
#9
Just straight wire it. Antennas arent impedence stable over large frequency ranges. Its close enough to 75 ohms.

Very expensive.

Its been done a million times before. Large C banders with Radio Shack Double Bowties at the focus. Works well.
 
#11
The only time i used any sort of parabolic antenna was to extend the range of my USB WiFi adapter to go more than 2 miles straight line-of-sight to the hotspot.

Of course, now that there's a huge WiFi WAN here now it's kinda pointless
 
#12
Says impedence is 50 OHMs. So how exactly do you hook it up to a TV? How to you go from 50 OHMs to 75 OHMs? Is there a 1.5:1 balun I'm not aware of?

Anyways seems to have the best gain of any antenna I've seen. So how much is one of those things?
75 ohm feeds are available as a special order. Last I knew, the cost was a little less than a grand.

It's one of the few antennas that retains it's gain when there is ice build-up.
 

BCF68

DTVUSA Member
#13
Last I knew, the cost was a little less than a grand..
Hardly seems worth it for a grand.

Its been done a million times before. Large C banders with Radio Shack Double Bowties at the focus. Works well.
Well I know it's been done I was just wondering what the gain levels are.

Kind of OT here but while I'm here. What if a house has aluminum siding? Wouldn't the sides of the house be one huge reflector? I mean if the one of the side was facing directly at the stations.

Also around here it seems metal roofs are making a comeback. I have a friend that needs his roof fixed and is think about going this route because it's cheaper. I'm pretty sure that would affect any indoor reception over a convential roof. Also though wouldn't the metal roof reflect singals so if one had an antenna mounted on such a roof that could potentially create multipath problems. Or as with the siding if the roof was facing the stations couldn't one use the relfectiveness of the roof to an advantage?
 
#14
Hardly seems worth it for a grand.

Or as with the siding if the roof was facing the stations couldn't one use the reflectiveness of the roof to an advantage?
I didn't pay a grand for my paraflector. I got it surplus.

To determine the exact interaction between a metal roof and terrain use this ray tracing program. www.arrl.org/notes/9876/HFTA.pdf

You will have to create the terrain profile of the roof manually. With most roof slopes the effect will be greater on VHF than UHF, but it really depends on the angles.

HFTA is a two dimensional ray tracing program. HFTA works best when the antenna is aimed directly down the slope of the roof. When at an angle, some of the reflected signal is skewed right or left, and HFTA doesn't consider the 3D reflection.

I have not tried HFTA on TV channels. I have used the old DOS version (called YT) up to TV channel 6 and it seemed to work fine.

By the way, the reason I used YT on TV channels was a attempt to understand the reputed disadvantage of low band VHF. What I learned was that the upward tilt of a channel 2 receiving antenna causes it to receive TV signals about 4 db worse than an antenna at the same height on channel 6. This is on top of the low band noise disadvantage of channel 2. Once again, the noise on channel 2 is about 4 db more than channel 6. My conclusion is that channels 2 and 3 are particularly bad for DTV reception. Channels 5 and 6 work far better.
 

EscapeVelocity

Moderator, , Webmaster of EV's Antenna Blog
#15
Yes, you are on the right thinking track with metal roofs.

However, more often than not, they cause more trouble than help....and you must know a bit about what you are doing, else its even more likely to be a problem.

Sorry, if "a million times" seemed a little snide. It hasnt been done a million times, but quite often by antenna hacks. I think the bowties are put a bit farther out than the focus at C band sat frequencies...for best results. But you can research this on the internet.
 

aerials

DTVUSA Jr. Member
#16
Parabolic UHF aerial

I have a Channel Master 4251 in storage. Antenna is larger than life. I have unfolded it and put it together and cant wait for the harsh Indiana winter to pass. Hold on for a few months and I will put it back up for testing. With some pictures. This type of antenna has a place in the digital fringe market.
 

Fringe Reception

Super Moderator, Chief Content Editor
Staff member
#17
re: your CM-4251

aerials,

I admit I am a wee bit envious. Do you want to sell it? I'd love to play (test) your antenna and in the process scare the H out of my neighbors above me with their picture windows ... they hated my 3-element CB vertical (later 10-meter ham horizontal) beam antenna right in their faces years ago and a comparatively small seven foot diameter screen would really 'annoy' them! PLEASE keep us updated and photos too.
Jim
 

aerials

DTVUSA Jr. Member
#20
Parabolic Uhf

aerials,

I admit I am a wee bit envious. Do you want to sell it? I'd love to play (test) your antenna and in the process scare the H out of my neighbors above me with their picture windows ... they hated my 3-element CB vertical (later 10-meter ham horizontal) beam antenna right in their faces years ago and a comparatively small seven foot diameter screen would really 'annoy' them! PLEASE keep us updated and photos too.
Jim
No I dont think i want to sell it but I am trying to rescue a Channel master 6 foot model from a school roof top.
 

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